The U-Boot README says fdt_addr_r is the right env var for fdt data
loaded into ram, but vendors also use fdtaddr and fdt_addr.  Check the
recommended variable first and fall back to the others.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273298 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f